A wide variety of cosmetic compositions for application to human skin make use of a structured liquid carrier to deliver colour or some other active material to the surface of the skin. Significant examples of such cosmetic compositions include antiperspirant or deodorant compositions which are widely used in order to enable their users to avoid or minimise wet patches on their skin, especially in axillary regions or to control or prevent the emission of malodours, which could otherwise arise when the user perspires. Other examples of cosmetic compositions include lip sticks.
Although structuring is a term that has often been employed in respect of materials which structure a carrier liquid, various other terms have been employed alternatively, including solidifying and gelling.
Antiperspirant or deodorant formulations have been provided with a range of different product forms. One of these is a so-called “stick” which is usually a bar of an apparently firm solid material held within a dispensing container and which retains its structural integrity and shape whilst being applied. In that respect they are representative of cosmetic compositions in stick form containing other active constituents. When a portion of the stick is drawn across the skin surface, a film of the stick composition is transferred to the skin surface. Although the stick has the appearance of a solid article capable of retaining its own shape for a period of time, the material often has a structured liquid phase so that a film of the composition is readily transferred from the stick to another surface upon contact.
Antiperspirant sticks can be divided into three categories. Suspension sticks contain a particulate antiperspirant active material suspended in a structured carrier liquid phase which often is anhydrous and/or in many instances may be water-immiscible. Emulsion sticks normally have a hydrophilic phase, commonly containing the antiperspirant active in solution, this phase forming an emulsion with a second, more hydrophobic, liquid phase. The continuous phase of the emulsion is structured. Solution sticks typically have the antiperspirant active dissolved in a structured liquid phase which is polar and may comprise a polar organic solvent, which is often water-miscible, and the polar phase can contain water.
There is substantial literature on structuring of cosmetic compositions, for example as represented by antiperspirant or deodorant compositions.
Conventionally, many sticks have been structured using naturally-occurring or synthetic waxy materials, in which term we include materials which resemble beeswax, in that they soften progressively with increase in temperature until they are fluid, generally by about 95° C. Examples of wax-structured sticks are described in an article in Cosmetics and Toiletries, 1990, Vol 105, P75-78, in U.S. Pat. Nos. 5,169,626 and 4,725,432 and in many other publications, in some of which such materials are called solidifying agents.
More specifically, it has been common practice for sticks to be structured or solidified by incorporating fatty alcohol into the composition, often accompanied by a smaller amount of castor wax. Sticks which are structured with fatty alcohol tend to leave visible white deposits on application to human skin; moreover the deposits can also transfer onto clothing when it comes into contact with the skin and the wearer can, for example, find white marks at the armhole of the sleeveless garment. Fatty alcohols are often regarded as coming within the general category of waxy materials, but we have observed that they are a more significant source of white deposits than various other waxy materials.
Some alternative structurants or solidifying agents to waxy materials have been proposed. For example, the use of dibenzylidene sorbitol (DBS) or derivatives thereof as gellant for a polar or hydrophylic carrier liquid has been proposed in a number of publications such as EP-A-512770, WO-92/19222, U.S. Pat. Nos. 4,954,333, 4,822,602 and 4,725,430. Formulations containing such gellants can suffer from a number of disadvantages, including instability in the presence of acidic antiperspirants, and comparatively high processing temperatures needed in the production of sticks.
Other alternative proposed structurants include various classes of esters or amides that are solid at ambient temperature and are capable of solidifying a hydrophobic or water-immiscible liquid carrier. One such class comprises ester or amide derivatives of 12-hydroxystearic acid, as described in inter alia U.S. Pat. No. 5,750,096. Another class of such esters or amides comprises N-acyl amino acid amides and esters, of which N-Lauroyl-L-glutamic acid di-n-butylamide is commercially available from Ajinomoto under their designation GP-1. They are described in U.S. Pat. No. 3,969,087. A further class which has been disclosed as gelling agents comprises the amide derivatives of di and tribasic carboxylic acids set forth in WO 98/27954 notably alkyl N,N′-dialkyl succinamides. Yet other amide structurants for water-immiscible liquid carriers are described in EP-A-1305604.
Although many amido-structurants have been identified already, it remains an objective to locate others which may meet the exacting standards of the cosmetic industry and ameliorate or eliminate one or more of the difficulties or disadvantages associated with the various amido structurants that have already been proposed or used.
One further class of compounds which contain a —CO—NH— group comprises cyclodipeptides, which are cyclic derivatives of aminoacids. Various cyclodipeptides has been described in an article by K Hanabusa et al entitled Cyclo(dipeptide)s as low molecular-mass Gelling Agents to harden Organic Fluids, J. Chem Soc. Commun., 1994 pp1401/2. The cyclodipeptides satisfied the general formula
in which R1 and R2 are designated organic residues. The selection exemplified included two materials (8 and 9) in which R1 represented alkyl esters, being either —CH2CO2CH2CH2CH2Me or —CH2CO2CH2CH2CHMeCH2CH2CH2CHMe2 and R2 represented —CH2Ph. This paper is herein referred to as Hanabusa I.
In an introductory section, Hanabusa states that the most difficult problem for the development of low molecular mass gelling agents is how to stabilise the formed gel, in other words how to prevent the transformation from the metastable gel to a crystalline state. Having conducted an extensive research programme into gels formed using low molecular mass gellants, the Applicants are able to confirm that the stabilisation of such gels often does indeed represent a serious and difficult problem, and indeed a problem that can be exacerbated in cosmetic compositions by the presence of other cosmetic ingredients. Hanabusa I subsequently makes a general assertion that the formed gels (sic. employing the exemplified cyclodipeptides in the list of organic fluids given in Table 1) were stable even after several months.
Various other cyclo(dipeptides) satisfying formula 1 above were described in a second article by Hanabusa et al entitled Low Molecular Weight Gelators for Organic Fluids: Gelation using a Family of Cyclo(dipeptide)s, in the Journal of Colloid and Interface Science 224, 231-244 (2000), herein called Hanabusa II. The text disclosed materials no 22 to 28, which were further esters like those of materials 8 and 9 in Hanabusa I, except that they were derived from different alkanols. Applicants have found that such esters, and in particular 27 and 28 in Hanabusa II, produced gels that were only poorly stable.
Applicants tested a number of materials as gellants in oils that are common in cosmetic formulations in accordance with Formula 1 in Hanabusa I, II-27 and II-28, which seemed to be Hanabusa's best gellants. Unfortunately, the resultant products demonstrated inferior storage characteristics, at laboratory ambient temperatures. Applicants deduced that at best, the capability of cyclodipeptides to gel organic fluid stably could vary significantly, depending on the chemical nature of the substituent residues R1 and R2.
A number of cyclic dipeptide derivatives have been described as gellants in Japanese Kokai No 2001-247451, in the name of Pola Chemical Industries Inc and Nisshin Oil Mills Ltd. These were either alkyl derivatives, which had already been described by Hanabusa or the unsubstituted cyclohexyl derivative which likewise showed inferior stability when tested in the same manner as those proposed by Hanabusa.